Copolymers from styrene and maleic anhydride are already known (SMA). These copolymers can be prepared by batch processes or in continuous processes. The first commercially available SMA copolymers were low-molecular weight products having molecular weights lower than 5000. These low molecular weight grades were used in, for instance, floor shampoos, floor waxes, emulsion paints and dispersing agents.
In addition, the later developed high-molecular SMA copolymers are important, and are used, for instance, in the automotive industry and in domestic articles.
As a result of the copolymerization of styrene with maleic anhydride, SMA copolymers have a greater dimensional stability under heat than polystyrene and styrene-acrylonitrile copolymers. This may be very advantageous. However, since SMA copolymers have a high softening temperature (approximately 220.degree. C.), it is difficult to process such copolymers through extrusion and injection moulding apparatus and the like. Moreover, the mechanical properties of such copolymers, such as impact resistance, are insufficient since SMA copolymers are very brittle. Incorporating a rubber in a SMA copolymers may improve impact-resistance characteristics while sacrificing other desired properties, such as, for instance, the rigidity of the copolymer.
A number of alleged solutions have already been proposed to overcome problems commonly associated with conventional SMA copolymers. For instance, British Pat. No. 1,315,219 describes mixtures consisting of a (i) copolymer of maleic anhydride, styrene and acrylonitrile, (ii) a copolymer from styrene and acrylonitrile and (iii) a graft copolymer of a mixture from styrene and acrylonitrile grafted on a rubber.
Another effort is described in British Pat. No. 1,316,330 as a mixture of a styrene-acrylonitrile copolymer with a styrene-maleic anhydride-acrylonitrile copolymer.
Another approach is described in U.S. Pat. No. 3,322,853 as a mixture consisting of a graft copolymer comprising a diene rubber substrate and hydroxylated substrate, a styrene-maleic anhydride copolymer having a low molecular weight and a polyepoxide. This mixture is composed of diene rubbers and polyepoxides and is used as a coating or adhesive. The UV resistance and oxidation stability of such a mixture are quite unsatisfactory.
Next, a still further proposal is disclosed in German patent application No. 3,140,565 now laid open for public inspection, which discloses a moulding compound consisting of 50 to 90% (wt) of a copolymer which consists of 65 to 90% (wt) of a monovinylaromatic monomer, 5 to 35% (wt) of an unsaturated dicarboxylic anhydride and of 10 to 50% (wt) of a rubber composed from a monomer from the group of the conjugated dienes, alkylacrylates, monovinyl aromatics, vinyl ethers of ethylene, propylene and a non-conjugated diene and further an ethyl-unsaturated monomer with an --OH, --SH or --NHR group. This moulding compound, however, has an undesirably low impact resistance.